Flyback converter is widely adopted for low power off-line applications, such as notebook adapter, cell phone charger etc. Quasi-resonant (QR) flyback is particularly popular because of less switching loss. It operates in discontinuous current conduction mode, and achieves zero voltage switching (ZVS) at low line and partial hard switching at high line. Recently there is growing need for higher power density for adapters, and high switching frequency design accompanied by efficiency improvement is necessary to pass thermal requirement in a smaller volume. QR flyback typically operates below 150 kHz, and switching loss becomes dominant when switching frequency further increases, especially at high line condition. Soft switching flyback converter were introduced to run at higher switching frequency (300˜1 MHz) with optimized efficiency. With new control method, QR flyback with synchronous rectifier (SR) can achieve ZVS for both low line and high line conditions. Before turning on the primary main switch, the SR is controlled to get inverse secondary side current, and then the inverse current is transferred to primary side to discharge parasitic capacitance of the primary main switch for ZVS. Active clamp flyback (ACF) is a two-switch topology that achieves soft switching. Operating in continuous current mode (CCM), or discontinuous current mode (DCM), it can achieve full ZVS, and furthermore, ACF recycles leakage inductance energy, which benefits both efficiency and device stress.
So far, soft switching flyback converter mentioned above is developed for fixed output voltage application. For wide range output applications such as USB PD adapter, ZVS may be lost, or efficiency may be degraded due to excessive circulation current.
Therefore, it is an urgent need at present to develop a control device and a control method that can overcome the above technical problems.
It is to be noted that the above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.